I believe all the solutions on this page are ground-referenced. That is, if there are long wires, less than ideal ground impedance, high ground currents or something they won't be precise.

But this can be eliminated by so-called <strong>Howland current pump</strong>.
See the following materials:

["Difference Amplifier Forms Heart of Precision Current Source"](http://www.analog.com/library/analogDialogue/cd/vol43n3.pdf#page=22)

["AN-1515 A Comprehensive Study of the Howland Current Pump"](http://www.ti.com/lit/an/snoa474a/snoa474a.pdf)

["Tame those versatile current source circuits"](http://e2e.ti.com/cfs-file/__key/telligent-evolution-components-attachments/00-18-01-00-01-18-14-27/Improved-Howland-Article.pdf)

["EEVblog #579 - Precision Low Current Source"](http://www.youtube.com/watch?v=OYOYI_IPKGY) (not Howland pump, but interesting)